Currently, a flying spot scanning apparatus based on a rotatable shielding mechanism is used for security check. The rotatable shielding mechanism is a circular cylinder, provided on its side wall with a helical line type gap for rays to be incident into and exiting the cylinder. During rotation of the circular cylinder, the ray, after passing through a slot collimator, irradiates the circular cylinder, and the radiated particle passes through the helical line shaped gap to form a flying spot. A scanning line is formed by high speed movement of the flying spots and is used to scan a moving object under detection. Such flying spot scanning apparatus can be used for non-destructive testing, security check, etc.
FIG. 1 is a use state diagram of a flying spot scanning apparatus. A slot collimator 3 is placed between a radiation source 1 and a shielding body 5 which is a hollow circular cylinder and is placed horizontally. An object 8 under detection is shown on the right, moving in a direction of the arrow 11. The shielding body 5 is provided on its side wall with a pair of helical line shaped gaps 6′, 6″. A ray radiated from the radiation source 1 passes through a line gap 2 on the slot collimator 3 and thus is constrained into a sector-shape beam of rays 4, and then is irradiated onto the circular cylinder 5. When the shielding body 5 rotates around its central axis (with a rotation direction as indicated by the arrow 12), the rays of the sector-shape beam of rays 4 incident from the gap 6′ pass through the gap 6″ and thus exit the cylinder (obviously, the positions of the gaps 6′ and 6″ correspond to each other), forming a pen-shape beam of rays 10. As the shielding body 5 continues to rotate, the flying spots exiting the gap 6″ form a plurality of pen-shape beams of rays. The object 8 under detection is moved in the direction as shown by the arrow 11 within the scanning range to complete a flying spot scanning.
It should be noted that process of forming flying spots is illustrated in principle in FIG. 1, reflecting the principle of forming flying spots, but in practical applications, the helical line shaped gaps 6′, 6″ on the shielding body 5 cannot be designed completely according to FIG. 1 since the rays radiated from the radiation source 1 are in a form of conical beam of rays with the focus of the radiation source as its circle center, rather than parallel rays. The collimated sector-shaped beam of rays with different opening angles passes through the shielding body 5. Therefore, the paths of the rays within the shielding body 5 are not parallel to one another, but are angled with respect to one another. Thus, if the gap 6′, 6″ are distributed along a direction of the full height of the shielding body 5 as shown in FIG. 1, definitely some of the gaps never receive any ray to pass through, and some incident rays are shielded and thus cannot exit the cylinder.
The shielding body of the flying spot forming apparatus that is used in practice is shown in FIG. 2, wherein the view on the left is a side view of the shielding body vertically placed, while the view on the right is a spread view of the side wall of the shielding body and the spread side wall is presented to be a rectangular plate having a certain thickness. In the view of the spread side wall, two gaps “he”, “h′e′” can be clearly seen wherein “he” is an incident groove, similar to the gap 6′, with the distribution thereof being limited within a circumferential range of 180 degrees; while “h′e” is an exit groove, similar to the gap 6″, with the distribution thereof being limited within another circumferential range of 180 degrees. At a certain time point, a ray radiated from the radiation source may be incident from a point on the incident groove “he” (such as a middle point of “he”) and exit from a corresponding point on the exit groove “h′e” (such as a middle point of “h′e′”) to form a flying spot, being consistent with the principle of forming flying spots.
However, in the flying spot scanning apparatus in FIG. 2, the incident groove “he” and the exit groove “h′e” each occupies a half of the space of the side wall and the exit groove “h′e” is distributed along a direction of the full height of the side wall. Such structure causes poor tension resistant properties of the side wall such that the shielding body is vulnerable to deformation during high speed rotation, and thus influences the scanning quality.